Holographic drive head alignments

ABSTRACT

Structures, subassemblies, devices, systems, and subsystems selected from: (1) a unitary holographic drive head assembly mounting structure; (2) an assembly including a unitary holographic drive head assembly mounting structure and a plurality of holographic drive head components and/or subassemblies; (3) a subassembly including a spatial light modulator, detector array, and a beam splitter; (4) a device including a spatial light modulator and a physical aperture positioned over or an imaged aperture projected onto the photoactive area of the spatial light modulator; (5) a system for optically aligning or pointing a laser in a holographic drive head assembly; (6) a light source subassembly including a laser, a fiber coupling lens; and an optical fiber having a fiber connector ready output end; and (7) a light source subsystem including a laser source, beam conditioning optics, fiber coupling optics for receiving the conditioned light beam, and a fiber optic connector for receiving the conditioned light beam from the fiber coupling optics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application makes reference to and claims the benefit of thefollowing co-pending U.S. Provisional Patent Application No. 60/684,531filed May 26, 2005. The entire disclosure and contents of the foregoingProvisional Application is hereby incorporated by reference. Thisapplication also makes reference to the following co-pending U.S. patentapplications. The first application is U.S. application Ser. No.11/440,370, entitled “Illuminative Treatment of Holographic Media,”filed May 25, 2006. The second application is U.S. application Ser. No.11/440,446, entitled “Methods and Systems for Laser Mode Stabilization,”filed May 25, 2006. The third application is U.S. application Ser. No.11/440,447, entitled “Phase Conjugate Reconstruction of Hologram,” filedMay 25, 2006. The fourth application is U.S. application Ser. No.11/440,448, entitled “Improved Operational Mode Performance of aHolographic Memory System,” filed May 25, 2006. The fifth application isU.S. application Ser. No. 11/440,359, entitled “Holographic Drive Headand Component Alignment,” filed May 25, 2006. The sixth application isU.S. application Ser. No. 11/440,358, entitled “Optical Delay Line inHolographic Drive,” filed May 25, 2006. The seventh application is U.S.application Ser. No. 11/440,357, entitled “Controlling the TransmissionAmplitude Profile of a Coherent Light Beam in a Holographic MemorySystem,” filed May 25, 2006. The eighth application is U.S. applicationSer. No. 11/440,372, entitled “Sensing Absolute Position of an EncodedObject,” filed May 25, 2006. The ninth application is U.S. applicationSer. No. 11/440,371, entitled “Sensing Potential Problems in aHolographic Memory System,” filed May 25, 2006. The tenth application isU.S. application Ser. No. 11/440,367, entitled “Post-Curing ofHolographic Media,” filed May 25, 2006. The eleventh application is U.S.application Ser. No. 11/440,366, entitled “Erasing Holographic Media,”filed May 25, 2006. The twelfth application is U.S. application Ser. No.11/440,365, entitled “Laser Mode Stabilization Using an Etalon,” filedMay 25, 2006. The thirteenth application is U.S. application Ser. No.11/440,368, entitled “Replacement and Alignment of Laser,” filed May 25,2006. The entire disclosure and contents of the foregoing U.S. patentapplications are hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

The present invention broadly relates to: (1) holographic drive head andcomponent alignment using a unitized mounting structure; (2) alignmentof certain holographic drive head components; and (3) replacement andalignment of the laser in a holographic drive head.

2. Related Art

Developers of information storage devices and methods continue to seekincreased storage capacity. As part of this development, holographicmemory systems have been suggested as alternatives to conventionalmemory devices. Holographic memory systems may be designed to recorddata one bit of information (i.e., bit-wise data storage). See McLeod etal. “Micro-Holographic Multi-Layer Optical Disk Data Storage,”International Symposium on Optical Memory and Optical Data Storage (July2005). Holographic memory systems may also be designed to record anarray of data that may be a 1-dimensional linear array (i.e., a 1×Narray, where N is the number linear data bits), or a 2-dimension arraycommonly referred to as a “page-wise” memory systems. Page-wise memorysystems may involve the storage and readout of an entire two-dimensionalrepresentation, e.g., a page of data. Typically, recording light passesthrough a two-dimensional array of dark and transparent areasrepresenting data, and the system stores, in three dimensions, the pagesof data holographically as patterns of varying refractive indeximprinted into a storage medium. See Psaltis et al., “HolographicMemories,” Scientific American, November 1995, where holographic systemsare discussed generally, including page-wise memory systems.

In a holographic data storage system, information is recorded by makingchanges to the physical (e.g., optical) and chemical characteristics ofthe holographic storage medium. These changes in the holographic mediumtake place in response to the local intensity of the recording light.That intensity is modulated by the interference between a data-bearingbeam (the data beam) and a non-data-bearing beam (the reference beam).The pattern created by the interference of the data beam and thereference beam forms a hologram which may then be recorded in theholographic medium. If the data-bearing beam is encoded by passing thedata beam through, for example, a spatial light modulator (SLM), thehologram(s) may be recorded in the holographic medium as an array oflight and dark squares or pixels. The holographic medium or at least therecorded portion thereof with these arrays of light and dark pixels maybe subsequently illuminated with a reference beam (sometimes referred toas a reconstruction beam) of the same or similar wavelength, phase,etc., so that the recorded data may be read.

Holographic data storage systems often comprise a holographic drive headassembly that is used in recording (writing) holograms to and to reading(reconstructing) holograms from a holographic storage medium. Thisholographic drive head assembly may comprise a variety of optical,optomechanical, optoelectrical and eletromechanical components. Forexample, these holographic drive head components may include lasers,beam splitters, lenses and lens arrays (e.g., Fourier Transformfocusing/storage lens, expander lenses, relays, scanner lens, etc.),phasemasks, encoders (e.g., spatial light modulators), detector arrays(e.g., cameras), waveplates, filters, mirrors, galvanometers (glavos),etc. Each of these holographic drive head components may be mounted asseparate components or may be combined into subassemblies comprising aplurality of components.

The design of such holographic drive head assemblies has traditionallybeen very complicated and complex. The individual components orsubassemblies of such components of the holographic head drive assemblymay be mounted on many square feet of a sophisticated optical.breadboard. The use of such large optical breadboards may benecessitated by the need to meticulously align the various componentsusing large sized optical test equipment. Because of the extremesensitivity of the optical alignment of such assemblies, a vibrationisolation system may be required for the optical breadboard. Meticulousalignment of the many components and elements of the holographic drivehead assembly may also be difficult and time consuming to achieve. Inaddition, the use of such large optical breadboards may be impracticalfor commercial holographic storage systems.

Previously, holographic drive head assemblies have also required precisealignment of the detector or sensor array (e.g., camera) for thereconstructed beam in multiple degrees of freedom to attain thenecessary relationship with the spatial light modulator (SLM). Thisalignment procedure may be time consuming and may necessitatecomplicated hardware to attain the level of adjustment needed ordesired. Realignment of this subassembly may also be required if thecamera, SLM or both need to be replaced because of failure, malfunction,damage, etc. In addition, reflections, for example, from bonding wires,pads, unused border pixels, etc., on the SLM may result in unwantednoise signals in the holograms that are written on the holographicstorage medium which may degrade the overall signal to noise ratio (SNR)of recovered holograms when read or reconstructed. Such reflections mayalso contribute to the problem of holographic drive head alignment.

Holographic drive head assemblies often use a primary laser, whichgenerates a primary coherent light beam. This primary coherent lightbeam may be split into a plurality beams (e.g., a data beam and areference beam) which are used, for example, to record/write hologramsto and to read/reconstruct holograms from the storage medium. It may benecessary to replace the primary laser due to laser failure,malfunction, damage, etc. Because the primary laser may be the firstoptical component in a chain of optical components or subassemblies ofthe holographic drive head assembly, replacing the primary laser mayrequire the realignment of all of the other components and/orsubassemblies of the holographic drive head assembly. This may be a verytedious and time consuming task to achieve. The optical path of theprimary laser may also require monitoring of its alignment. The primarycoherent light beam generated by the laser may also be susceptible topointing errors due changes in the ambient temperature, wavelength,electrical current supplied, etc.

Accordingly, what may be needed are ways to: (1) more easily align thevarious components and/or subassemblies of a holographic drive headassembly; (2) reduce the size of the structure for mounting thesevarious components and/or subassemblies of a holographic drive headassembly; (3) provide a mounting structure for the various componentsand/or subassemblies which minimizes, reduces, eliminates, etc.,vibrations that may affect the optical alignment of the holographicdrive head assembly; (4) more easily and precisely align the camera withthe SLM, as well as to allow for easy replacement thereof without theneed of realignment; (5) minimize, reduce, eliminate, etc., reflectionson the SLM; and (6) more easily replace the primary laser withoutrequiring tedious and timing consuming realignment of the othercomponents and/or subassemblies of the holographic drive head assembly.

SUMMARY

According to a first broad aspect of the present invention, there isprovided an article comprising

-   -   a unitary holographic drive head assembly mounting structure        having sufficient rigidity to minimize motion effects on optical        alignment of holographic drive head assembly components and/or        subassemblies mounted on the mounting structure;    -   the mounting structure having a plurality of preselected        locations for mounting holographic drive head components and/or        subassemblies requiring optical alignment; and    -   passive alignment means associated with one or more of the        preselected locations for optically aligning the holographic        drive head components and/or subassemblies.

According to a second broad aspect of the present invention, there isprovided an assembly comprising:

-   -   a unitary holographic drive head assembly mounting structure        having:        -   sufficient rigidity to minimize motion effects on optical            alignment of holographic drive head assembly components            and/or subassemblies mounted on the mounting structure;        -   a plurality of preselected locations for mounting            holographic drive head components and/or subassemblies            requiring optical alignment; and        -   passive alignment means associated with one or more of the            preselected locations for optically aligning the holographic            drive head components and/or subassemblies; and    -   a plurality of holographic drive head components and/or        subassemblies, each of the holographic drive head components        and/or subassemblies being mounted at one of the preselected        locations.

According to a third broad aspect of the present invention, there isprovided a unitized subassembly comprising:

-   -   a spatial light modulator;    -   a detector array;    -   a beam splitter; and    -   means for associating the spatial light modulator and the        detector array to the beam splitter so that the spatial light        modulator and the detector array are optically and mechanically        aligned with respect to each other.

According to a fourth broad aspect of the present invention, there isprovided a device comprising:

-   -   a spatial light modulator having a photoactive area; and    -   a physical aperture having high absorption positioned over the        photoactive area or an imaged aperture imaged onto the        photoactive area;    -   wherein the physical or imaged aperture reduces reflections in        the photoactive area which cause degradation of the signal to        noise ratio of a recorded hologram.

According to a fifth broad aspect of the present invention, there isprovided a system comprising one or more of the following components foroptically aligning or pointing a laser in a holographic drive headassembly with respect to an optical path:

-   -   a pair of independent optical path bending or altering elements        positioned in the optical path after the laser and before the        next optical component or subassembly in the optical path;    -   a spatial filter having associated therewith a pinhole through        which light generated by the laser may pass, wherein the amount        of light passing through the pinhole is used to determine the        optical alignment of the laser with respect to the optical path;    -   one or more alignment apertures positioned in an optical path        after the laser, wherein the alignment apertures are used to        determine the optical alignment of the laser with respect to the        optical path; or    -   means for splitting off a portion of the light generated by the        laser to provide a monitoring beam, and means for analyzing the        monitoring beam to determine whether the laser is optically        aligned or pointing correctly.

According to a sixth broad aspect of the present invention, there isprovided a laser light source subassembly comprising:

-   -   a laser;    -   a fiber coupling lens connected to the laser; and    -   an optical fiber having an input end connected to the fiber        coupling lens and a fiber connector ready output end.

According to a seventh broad aspect of the present invention, there isprovided a light source subsystem comprising:

-   -   a laser source providing a light beam;    -   beam conditioning optics for conditioning the light beam to        provide a conditioned light beam;    -   fiber coupling optics for receiving the conditioned light beam;        and    -   a fiber optic connector having an input end and an output end,        wherein the input end receives the conditioned light beam from        the fiber coupling optics.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic block diagram of an exemplary holographic memorysystem which may include one or more embodiments of the presentinvention;

FIG. 2A is an architectural block diagram of the components of aholographic memory system illustrating the optical paths used during awrite or record operation, and which may include one or more embodimentsof the present invention;

FIG. 2B is an architectural block diagram of the components of aholographic memory system illustrating the optical paths used during aread or reconstruct operation, and which may include one or moreembodiments of the present invention;

FIG. 3 is a top perspective view of a mounting structure showing anembodiment of a means for passive alignment of an optical component orsubassembly according to the present invention;

FIG. 4 is a top perspective view of another embodiment of the presentinvention for passive alignment of an optical component or subassembly;

FIG. 5 is a top perspective view of an embodiment of a pair ofsubassemblies which are passively aligned according to the presentinvention;

FIG. 6 is a top perspective view of an embodiment of a mountingstructure with preselected locations and passive alignment members formounting holographic drive head components and/or subassemblies;

FIG. 7 is a bottom perspective view of the embodiment of the mountingstructure shown in FIG. 6;

FIG. 8 is a top perspective view an embodiment of a mounting block whichmay be used with the mounting structure of FIG. 6 showing the mountingor referencing surfaces for a storage lens subassembly;

FIG. 9 is a different perspective view of the mounting block of FIG. 8showing the mounting or referencing surfaces for a scanner lenssubassembly;

FIG. 10 is a top perspective view of the mounting structure of FIG. 6showing some of the holographic drive head components, subassemblies andmounting block of FIGS. 8-9 mounted thereon;

FIG. 11 is a different top perspective view of the mounting structure ofFIG. 10 showing the holographic drive head components, subassemblies andmounting block of FIGS. 8-9 mounted thereon;

FIG. 12 is a bottom perspective view of the mounting structure similarto that of FIG. 7 showing the holographic drive head components andsubassemblies mounted thereon;

FIG. 13 is a perspective view of an embodiment of a partial subassemblyaccording to the present invention of a passively aligned spatial lightmodulator (SLM) and a polarizing beam splitter;

FIG. 14 is a perspective view of an embodiment of a completedsubassembly according to the present invention of a passively alignedspatial light modulator (SLM), polarizing beam splitter and camera;

FIG. 15 is a top perspective view showing a high absorption physicalaperture which is positioned over the photoactive area of a spatiallight modulator;

FIG. 16 is a portion of the block diagram of FIG. 2A illustratingpotential locations for laser alignment elements according to anembodiment of the present invention;

FIG. 17 is a schematic view illustrating the embodiment of FIG. 16 foraligning the primary laser using two alignment elements, each having analignment aperture;

FIG. 18 is a schematic illustration of a laser light source subassemblyaccording to an embodiment of the present invention; and

FIG. 19 is a schematic block diagram of an embodiment of a fiber opticcoupled laser light source subsystem according to the present inventionwhich may be used in the holographic memory system illustrated in FIG.2A.

DETAILED DESCRIPTION

It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

Definitions

Where the definition of terms departs from the commonly used meaning ofthe term, applicant intends to utilize the definitions provided below,unless specifically indicated.

For the purposes of the present invention, directional terms such as“top”, “bottom”, “above”, “below”, “left”, “right”, “horizontal”,“vertical”, etc. are merely used for convenience in describing thevarious embodiments of the present invention. The embodiments of thepresent invention may be oriented in various ways. For example, theembodiments shown in FIGS. 3 through 12 may be flipped over, rotated by90° in any direction, etc.

For the purposes of the present invention, the term “laser” refers toconventional lasers, as well as laser diodes (LDs).

For the purposes of the present invention, the term “light source”refers to any source of electromagnetic radiation of any wavelength, forexample, from a laser, etc. Suitable light sources for use inembodiments of the present invention include, but are not limited to,those obtained by conventional laser sources, e.g., the blue and greenlines of Ar⁺ (458, 488, 514 nm) and He—Cd lasers (442 nm), the greenline of frequency doubled YAG lasers (532 nm), and the red lines ofHe—Ne (633 nm), Kr⁺lasers (647 and 676 nm), and various laser diodes(LDs) (e.g., emitting light having wavelengths of from 290 to 900 nm).

For the purposes of the present invention, the term “spatial lightintensity” refers to a light intensity distribution or pattern ofvarying light intensity within a given volume of space.

For the purposes of the present invention, the terms “holographicgrating,” “holograph” or “hologram” (collectively and interchangeablyreferred to hereafter as “hologram”) are used in the conventional senseof referring to an interference pattern formed when a signal beam and areference beam interfere with each other. In cases wherein digital datais recorded, the signal beam may be encoded with a data modulator, e.g.,a spatial light modulator, etc.

For the purposes of the present invention, the term “holographicrecording” refers to the act of recording a hologram in a holographicrecording medium.

For the purposes of the present invention, the term “multiplexingholograms” refers to recording, storing, etc., a plurality of hologramsin the same volume or nearly the same volume of the holographicrecording medium by varying a recording parameter(s) including, but notlimited to, angle, wavelength, phase code, shift, correlation,peristrophic, etc. The multiplexed holograms that are recorded, stored,etc., may be read, retrieved, reconstructed, etc., by using the samerecording parameter(s) used to record, store, etc., the respectiveholograms.

For the purposes of the present invention, the term “holographicrecording medium” refers to a component, material, etc., that is capableof recording and storing, in three dimensions (i.e., the X, Y and Zdimensions), one or more holograms as one or more pages as patterns ofvarying refractive index imprinted into the medium. Examples ofholographic media useful herein include, but are not limited to, thosedescribed in: U.S. Pat. No. 6,103,454 (Dhar et al.), issued Aug. 15,2000; U.S. Pat. No. 6,482,551 (Dhar et al.), issued Nov. 19, 2002; U.S.Pat. No. 6,650,447 (Curtis et al.), issued Nov. 18, 2003, U.S. Pat. No.6,743,552 (Setthachayanon et al.), issued Jun. 1, 2004; U.S. Pat. No.6,765,061 (Dhar et al), Jul. 20, 2004; U.S. Pat. No. 6,780,546 (Trentleret al.), issued Aug. 24, 2004; U.S. Patent Application No. 2003-0206320,published Nov. 6, 2003, (Cole et al), and U.S. Patent Application No.2004-0027625, published Feb. 12, 2004, the entire contents anddisclosures of which are herein incorporated by reference.

For the purposes of the present invention, the term “data page” or“page” refers to the conventional meaning of data page as used withrespect to holography. For example, a data page may be a page of data(i.e., two-dimensional assembly of data), one or more pictures, etc., tobe recorded in a holographic recording medium.

For the purposes of the present invention, the term “recording light”refers to a light source used to record into a holographic recordingmedium. The spatial light intensity pattern of the recording light iswhat is recorded.

For the purposes of the present invention, the term “recording data”refers to storing or writing holographic data in a holographic medium.

For the purposes of the present invention, the term “reading data”refers to retrieving, recovering, or reconstructing holographic datastored in a holographic medium.

For the purposes of the present invention, the term “data modulator”refers to any device that is capable of optically representing data inone or two-dimensions from a signal beam.

For the purposes of the present invention, the term “spatial lightmodulator” (SLM) refers to a data modulator device that is anelectronically controlled, active optical element.

For the purposes of the present invention, the term “refractive indexprofile” refers to a two-dimensional (X, Y) mapping of the refractiveindex pattern recorded in a holographic recording medium.

For the purposes of the present invention, the term “data beam” refersto a recording beam containing a data signal. As used herein, the term“data modulated beam” refers to a data beam that has been modulated by amodulator such as a spatial light modulator (SLM).

For the purposes of the present invention, the term “optical element”refers to any component or plurality of components that affect the phaseof the light, including, but not limited to, the spatial location of thelight, the angle of the light, etc.

For the purposes of the present invention, the term “reflective opticalelement” refers to an optical element that reflects incident light.

For the purposes of the present invention, the term “transmissiveoptical element” refers to an optical element that allows incident lightto pass therethrough.

For the purposes of the present invention, the term “noise” refers toany undesirable optical signal that may cause a loss in the desiredfidelity of the signal, including, but not limited to, coherent noise,incoherent noise, interference fringes, etc.

For the purposes of the present invention, the term “assembly” refers toan association or combination of one or more individual components orsubassemblies.

For the purposes of the present invention, the term “subassembly” refersto an association or combination of one or more individual componentswhich function as a unitary or unified component.

For the purposes of the present invention, the term “component” refersto an individual element, structure, device, etc.

For the purposes of the present invention, the term “holographic drivehead assembly” refers to those components, subassemblies, mountingstructures, etc., of a holographic memory system which are primarilyused to record (write) holograms to and/or read (reconstruct) hologramsfrom the holographic storage medium. The components, subassemblies, etc,of the holographic drive head assembly may include lasers, beamsplitters, lenses and lens arrays (e.g., Fourier Transformfocusing/storage lens, expander lenses, relays, scanner lens, etc.),phasemasks, encoders (e.g., spatial light modulators), detector arrays(e.g., cameras), waveplates, filters, mirrors, galvanometers (glavos),etc., as well as mounting structures, passive alignment elements, etc.,for these components and subassemblies.

For the purposes of the present invention, the terms “unitary,”“unified” and “unitized” refer to a structure, component or subassemblywhich functions as a single unit.

For the purposes of the present invention, the term “rigidity” refers tothe inflexibility, firmness, stiffness, etc., of a structure, material,etc., which is stiff, unbending, firm, unyielding, etc.

For the purposes of the present invention, the term “motion effects”refer to movement effects caused by external or internal physicalvibrations, shaking, shocks, impacts, etc.

For the purposes of the present invention, the terms “optical alignment”and “optically aligned” refer to alignment of components, subassemblies,etc., relative to the optical path.

For the purposes of the present invention, the term “preselectedlocation” refers to a predetermined position, place, spot, etc., wherecomponents, subassemblies, etc., are to be or may be mounted in theholographic drive head assembly.

For the purposes of the present invention, the term “plurality” refersto one or more, subassemblies, components, elements, locations, etc.,and includes the term multiplicity.

For the purposes of the present invention, the term “passive alignmentmeans” and “passive alignment members” refer to means or members thatoptically align or maintain optical alignment of components,subassemblies, etc., of the holographic drive head assembly, and/orwhich restrict undesired motion or movement thereof in one or morespecific directions, without active adjustment, movement, motion, etc.Passive alignment members may include defined spaces, such as slots,holes, recesses, grooves, channels, datums, etc., physical elements suchas rails, guides, walls, surfaces, mounting blocks, pins, bosses, etc.,other passive alignment features, etc., or any combination thereof. Thepassive alignment members may be of any suitable shape, size,orientation, etc., including V-shaped, C-shaped, L-shaped, U-shaped,cylindrical-shaped, rectangular or box-shaped, squared-shaped, etc.Combinations of passive alignment members may also be used to align ormaintain alignment. For example, a plurality of connected or unconnectedsurfaces which, together, define a particular orientation, shape,configuration, etc., which align or maintain alignment may be used incombination as passive alignment members.

For the purposes of the present invention, the terms “a detector array”or “sensor array” (hereinafter collectively referred to as “detectorarray”) refer to a device which is capable of determining the positionof an incident beam or which provides information for determining theposition of an incident beam. Detector arrays may include a quad cell, apositioning sensing detector, a camera, etc.

For the purposes of the present invention, the term “optical divider”refers to an optical component or assembly which divides or splits alight beam into at least one or more separate light beams withcontrolled and possibly adjustable intensities. Optical dividers includebeam splitters which may also polarize the split beams, i.e., may be apolarizing beam splitter.

For the purposes of the present invention, the term “means forassociating the spatial light modulator and the detector array to thebeam splitter” refer to means, members, etc., that associate, attach,secure, mount, etc., the spatial light modulator and the detector arrayto the beam splitter so that the spatial light modulator and thedetector array are optically aligned with respect to each other and maybe assembled and aligned into a holographic storage device withoutadditional relative adjustment, movement, motion, etc., between thespatial light modulator, detector array, or beam splitter Associationmeans may include adhesive, bonding agents, mechanical fasteners (e.g.,bolts, etc.), mechanical mounting structures, components, elements,frames, or parts (e.g., brackets, blocks, etc.), etc.

For the purposes of the present invention, the term “physical aperture”refers to an aperture formed, defined, made, etc., in a physicalcomponent, element, etc.

For the purposes of the present invention, the term “imaged aperture”refers to an image of a light beam as it passes through an aperturewhich is created, projected, etc., onto a surface, component, element,etc. Imaged apertures may be formed and imaged, projected, etc., onto asurface, component, element, etc., by methods, devices, etc., used toform and image, project, etc., phasemasks onto a surface, component,element, etc.

For the purposes of the present invention, the term “aperture havinghigh absorption” refers to an aperture having a coating, material,texture, etc., or any combination thereof which absorbs substantiallyall incident light, for example, a sand blasted aluminum with blackanodized coating, a black oxide coating on steel, black felt, etc.

For the purposes of the present invention, the term “pinhole” refers torelatively small or fine diameter hole formed, for example, by drilling(e.g., laser drilling), etching, etc. in an element, component, etc. Thepinhole may have a diameter of from about 0.5 to about 1000 micrometers,more typically from about 5 to about 25 micrometers.

For the purposes of the present invention, the term “spatial filter”refers to a lens or lens assembly used in conjunction with a pinholewherein the pinhole is centered on and placed at the focal point of thelens or lens assembly so as to filter the angular spectrum of anincident light beam so that the light beam approximates a point sourceof light.

For the purposes of the present invention, the term “optical path”refers to the nominal path a light beam is designed to travel as itpropagates between and/or through various optical components and/orsubassemblies, for example, lenses, mirrors, prisms, beam splitters,etc.

For the purposes of the present invention, the term “pointing” withregard to a laser or laser beam refers to the angular direction in whichthe laser beam propagates through space.

For the purposes of the present invention, the terms “fiber couplingoptics” or “fiber coupling lens” refer to a lens or lens assembly whichis capable of, or used for, focusing a light beam onto the end of anoptical fiber placed at the focal point of that lens or lens assemblyand with the appropriate numerical aperture to be coupled into theoptical fiber. The numerical aperture may be defined as the sine of halfthe cone angle of the focused light beam.

For the purposes of the present invention, the term “optical fiber”refers to a flexible filament of glass which may be surrounded by acladding of material with a different index of refraction so as to causea portion or all of the light rays incident on the end of the opticalfiber (and within a particular numerical aperture) to propagate alongthe optical fiber via total internal reflection. This numerical aperture(NA) of an optical fiber may be defined in terms of the indices ofrefraction of the core and cladding of the fiber. Specifically, theacceptance NA may be defined as the square root of the differencebetween the square of the core refractive index (CoreRI) and the squareof the cladding refractive index (CladRI), or in other wordsNA=[(CoreRI)²−(CladRI)²]^(1/2).

For the purposes of the present invention, the term “input end” refersto the end of an optical fiber wherein light rays enter the fiber, whilethe “output end” refers to the end of the optical fiber from which lightrays exit the fiber.

For the purposes of the present invention, the term “fiber connectorready output end” refers to an output end of an optical fiber which isencased in a standard fiber optic connector such as, for example, FC-PC,FC-APC, FC-ST, F-SMA, etc.

For the purposes of the present invention, the term “mode” refers to thenumber of local maxima of the electric and magnetic fields in thedirection of oscillation, transverse to light beam.

For the purposes of the present invention, the term “single-mode” refersto a light beam with electric and magnetic field components having onlya single local maximum in the transverse directions, also known as TEM00light beams.

Description of Holographic Memory System Generally

FIG. 1 is a block diagram of an exemplary holographic memory system inwhich embodiments of the present invention may be used. Althoughembodiments of the present invention may be described in the context ofthe exemplary holographic memory system shown in FIG. 1, the presentinvention may also be implemented in connection with any system now orlater developed that implements holographics.

Holographic memory system 100 (“HMS 100” herein) receives along signalline 118 signals transmitted by an external processor 120 to read andwrite data to a photosensitive holographic storage medium 106. As shownin FIG. 1 processor 120 communicates with drive electronics 108 of HMS100. Processor 120 transmits signals based on the desired mode ofoperation of HMS 100. For ease of description, the present inventionwill be described with reference to read and write operations of aholographic system. However, that the present invention may be appliedto other operational modes of a holographic system, such as Pre-Cure,Post-Cure, Write Verify, or any other operational mode implemented nowor in the future in an holographic system.

Using control and data information from processor 120, drive electronics108 transmit signals along signal lines 116 to various components of HMS100. One such component that may receive signals from drive electronics108 is coherent light source 102. Coherent light source 102 may be anylight source known or used in the art that produces a coherent lightbeam. In one embodiment, coherent light source 102 may be a laser.

The coherent light beam from coherent light source 102 is directed alonglight path 112 into an optical steering subsystem 104. Optical steeringsubsystem 104 directs one or more coherent light beams along one or morelight paths 114 to holographic storage medium 106. In the writeoperational mode described further below at least two coherent lightbeams are transmitted along light paths 114 to create an interferencepattern in holographic storage medium 106. The interference patterninduces alterations in storage medium 106 to form a hologram.

In the read operational mode, holographically-stored data is retrievedfrom holographic storage medium 106 by projecting a reconstruction orprobe beam along light path 114 into storage medium 106. The hologramand the reconstruction beam interact to reconstruct the data beam whichis transmitted along light path 122. The reconstructed data beam may bedetected by a sensor 110. Sensor 110 may be any type of detector knownor used in the art. In one embodiment, sensor 110 may be a camera. Inanother embodiment, sensor 110 may be a photodetector.

The light detected at sensor array 110 is converted to a signal andtransmitted to drive electronics 108 via signal line 124. Processor 120then receives the requested data or related information from driveelectronics 108 via signal line 118.

The components of an exemplary embodiment of HMS 100 are illustrated inmore detail in FIGS. 2A and 2B, and is referred to generally asholographic memory system 200 (“HMS 200” herein). FIGS. 2A and 2B aresimilar schematic block diagrams of the components of one embodiment ofHMS 200 illustrating the optical paths utilized during write and readoperations, respectively.

Referring first to FIG. 2A, HMS 200 is shown in a record or writeoperation or mode (herein “write mode configuration”). Coherent lightsource 102 (see FIG. 1) is shown in FIG. 2A in the form of laser 204.Laser 204 receives via signal line 116 control signals from anembodiment of drive electronics 108 (FIG. 1), referred to in FIG. 2A asdrive electronics 202. In the illustrated write mode configuration, sucha control signal may cause laser 204 to generate a coherent light beam201 which is directed along light path 112 (see FIG. 1).

Coherent light beam 201 from laser 204 is reflected by mirror 290 andmay be directed through optical shutter 276. Optical shutter 276comprises beam deviation assembly 272, focusing lens 274 and pinhole 206that collectively shutter coherent light beam 201 from entering theremainder of optical steering subsystem 104. The details of theexemplary optical shutter 276 are described in more detail in theabove-related U.S. application Ser. No. 11/440,448, entitled “ImprovedOperational Mode Performance of a Holographic Data Storage (HDS) DriveSystem,” filed May 25, 2006. Further, it should be noted that this isbut one exemplary optical shutter and other embodiments may use adifferent type of optical shutter or an optical shutter need not beused.

Coherent light beam 201 passing through optical shutter 276 enters mainexpander assembly 212. Main expander assembly 212 includes lenses 203and 205 to expand coherent light beam 201 to a fixed diameter and tospatially filter coherent light beam 201. Main expander assembly 212also includes lens 274 and pinhole 206 to spatially filter the lightbeam. An exposure shutter 208 within main expander assembly 212 is anelectromechanical device which may be used to control recording exposuretimes.

Upon exiting main expander assembly 212, the coherent light beam 201 maybe directed through apodizer 210. Light emitted from a laser such aslaser 204 may have a spatially varying distribution of light. Apodizer210 converts this spatially varying intensity beam 201 from laser 204into a more uniform beam with controlled edge profiles.

After passing through apodizer 210, coherent light beam 201 may entervariable optical divider 214. Variable optical divider 214 uses adynamically-controlled polarization device 218 and at least onepolarizing beam splitter (PBS) 216 to redirect coherent light beam 201into one or more discrete light beams transmitted along two light paths114 (see FIG. 1), referred to in FIG. 2A as light path 260 and lightpath 262. Variable optical divider 214 dynamically allocates power ofcoherent light beam 201 among these discrete light beams, indicated as280 and 282. In the write operational mode shown in FIG. 2A, thediscrete light beam directed along light path 260 is referred to asreference light beam 280 (also referred to herein as reference beam280), while the discrete light beam directed along light path 262 isreferred to as data light beam 282 (also referred to herein as data beam282).

Upon exiting variable optical divider 214, reference beam 280 isreflected by mirror 291 and directed through a beam shaping device 254A.After passing through beam shaping device 254A, reference beam 280 isreflected by mirrors 292 and 293 towards galvo mirror 252. Galvo mirror252 reflects reference beam 280 into scanner lens assembly 250. Scannerlens assembly 250 has lenses 219, 221, 223 and 225 to pivotally directreference beam 280 at holographic storage medium 106, shown in FIG. 2Aas holographic storage disk 238.

Referring again to variable optical divider 214, data light beam 282exits variable optical divider 214 and passes through data beam expanderlens assembly 220. Data beam expander 220 implements lenses 207 and 209to magnify data beam 282 to a diameter suitable for illuminating SpatialLight Modulator (SLM) 226, located further along data beam path 262.Data beam 282 then passes through phasemask 222 to improve theuniformity of the Fourier transform intensity distribution. Data beam282 illumination of phasemask 222 is then imaged onto SLM 226 via 1:1relay 224 having lenses 211 and 213. PBS 258 directs data beam 282 ontoSLM 226.

SLM 226 modulates data beam 282 to encode information into data beam282. SLM 226 receives the encoding information from drive electronics202 via a signal line 116. Modulated data beam 282 is reflected from SLM226 and passes through PBS 258 to a switchable half-wave plate 230.Switchable half-wave plate 230 may be used to optionally rotate thepolarization of data beam 282 by 180 degrees. A 1:1 relay 232 containinga beam-shaping device 254B and lenses 215 and 217 directs data beam 282to storage lens 236 which produces a filtered Fourier transform of theSLM data inside holographic storage disk 238. At a particular pointwithin holographic storage disk 238, reference light beam 280 and datalight beam 282 create an interference pattern to record a hologram inholographic storage disk 238.

Referring next to the read mode configuration illustrated in FIG. 2B,laser 204 generates coherent light 201 in response to control signalsreceived from drive electronics 202. As noted with regard to FIG. 2A,coherent light beam 201 is reflected by mirror 290 through opticalshutter 276 that shutters coherent light beam 201 from entering theremainder of optical steering subsystem 104. Coherent light beam 201thereafter enters main expander assembly 212 which expands and spatiallyfilters the light beam, as described above with reference to FIG. 2A.Upon exiting main expander assembly 212, coherent light beam 201 isdirected through apodizer 210 to convert the spatially varying intensitybeam into a more uniform beam.

In the arrangement of FIG. 2B, when coherent light beam 201 entersvariable optical divider 214, dynamically-controlled polarization device218 and PBS 216 collectively redirect the coherent light into onediscrete light beam 114, referred to as reconstruction beam 284.Reconstruction beam 284 travels along reconstruction beam path 268,which is the same path 260 traveled by reference beam 280 during thewrite mode of operation, as described with reference to FIG. 2A.

A desired portion of the power of coherent light beam 201 is allocatedto this single discrete reconstruction beam 284 based on the selectedpolarization implemented in device 218. In certain embodiments, all ofthe power of coherent light beam 201 is allocated to reconstructionlight beam 284 to maximize the speed at which data may be read fromholographic storage disk 238.

Upon exiting variable optical divider 214, reconstruction beam 284 isreflected from mirror 291. Mirror 291 directs reconstruction beam 284through beam shaping device 254A. After passing through beam shapingdevice 254A, reconstruction beam 284 is directed to scanner lensassembly 250 by mirrors 292 and 293, and galvo 252. Scanner lensassembly 250 pivots reconstruction beam 284 at a desired angle towardholographic storage disk 238.

During the read mode, reconstruction beam 284 may pass throughholographic storage disk 238 and may be retro-reflected back through themedium by a second conjugator galvo 240. As shown in FIG. 2B, the datareconstructed on this second pass through storage disk 238 is directedalong reconstructed data beam path 298 as reconstructed data beam 264.

Reconstructed data beam 264 passes through storage lens 236 and 1:1relay 232 to switchable half wave plate 230. Switchable half wave plate230 is controlled by drive electronics 202 so as to have a negligiblepolarization effect. Reconstructed data beam 264 then travels throughswitchable half wave plate 230 to PBS 258, all of which are describedabove with reference to FIG. 2A. PBS 258 reflects reconstructed databeam 264 to an embodiment of sensor 110 (see FIG. 1) in the form of acamera 228. The light detected by camera 228 is converted to a signaland transmitted to drive electronics 202 via signal line 124 (see FIG.1). Processor 120 then receives the requested data and/or relatedinformation from drive electronics 202 via signal line 118 (see FIG. 1).

HMS 200 may further comprise an illuminative media cure subsystem 242.Media cure subsystem 242 is configured to provide a uniform curing beamwith reduced coherence to storage disk 238 to pre-cure and/or post-curea region of storage disk 238 following the writing process. Media curesubsystem 242 may comprise a laser 256 sequentially aligned with adiffuser 244, a lenslet array 243 and a lens 229. The light from laser256 is processed by diffuser 244, lenslet array 243, and lens 229 toprovide a uniform curing beam with reduced coherence prior to reachingstorage disk 238.

HMS 200 may additionally comprise an associative read after write (ARAW)subsystem 248. ARAW subsystem 248 is configured to partially verify ahologram soon after the hologram is written to holographic storage disk238. ARAW subsystem may comprise a lens 227 and a detector 246.Holographic system 100 uses ARAW subsystem 248 by illuminating a writtenhologram with an all-white data page. When a hologram is illuminated bythis all-white data page, ARAW subsystem 248 detects the reconstructedreference beam resulting from this all-white illumination. Specifically,detector 246 examines the reconstructed reference beam to verify thatthe hologram has been recorded correctly.

Description of Mounting Structure for Holographic Drive Head Assembly

Embodiments of the present invention relate to a mounting structure forthe holographic drive head assembly. The mechanical stability of aholographic drive head assembly may be greatly improved through the useof a unitary (e.g., monolithic) mounting structure having sufficientrigidity to minimize, reduce, diminish, eliminate, etc., some, most orall motion effects (e.g., those caused by vibrations, shocks, impacts,etc.) which might affect optical alignment of holographic drive headcomponents and/or subassemblies mounted thereon. Mounting structuresmade from or comprising a plurality of sections, portions, etc., may bemore likely to have residual manufacturing induced stresses which maytend to warp and twist the structure over time and/or due to temperaturevariations as those stresses relax. Mounting of some, most or all of theoptical, optomechanical, optoelectrical, and electromechanicalcomponents, and/or subassemblies of such components, of the holographicdrive head assembly on a unitary (e.g., monolithic) mounting structuremay be used improve optical alignment, thermal stability, mechanicalstiffness, etc., of the holographic drive head assembly. Such componentsand/or subassemblies may include, for example, laser 204, data beamexpander subassembly 220, relay subassemblies 224 and 232, scanner lenssubassembly 250, storage lens subassembly 236, SLM/PBS subassembly226/258, etc. Additionally, a unitary mounting structure may be made,formed, manufactured, etc., with better positional tolerances of passivealignment features than might be achieved with mounting structures madefrom or comprising a plurality of sections, portions, etc., of equalexpense.

This mounting structure may be made, formed, manufactured, etc., from arelatively uncomplex structure (e.g., a large plate), or may be made orformed as a more complex (e.g., three dimensional) structure. Themounting structure may be made, formed, manufactured, etc., throughmachining, casting, molding (e.g., injection molding), etc., or anycombination thereof. The mounting structure may be formed, made,manufactured, etc., in such a way and/or from such a material so as tohave very stable mechanical properties having, for example, minimaltwist of the structure over time and/or as a result of temperaturevariations. The mounting structure may be formed, made, manufactured,etc., from any material which provides sufficient rigidity to minimize,reduce, diminish, eliminate, etc., some, most or all motion effects(e.g., vibrations, shocks, impacts, etc.) affecting optical alignment ofholographic drive head assembly components and/or subassemblies. Forexample, the mounting structure may comprise reinforced plastics, carboncomposites, light weight metals such as aluminum, magnesium, etc.

The stability of the mounting structure may be further improved, and themass further reduced, by selective removal or omission of non-criticalvolumes of material therefrom. Non-critical volumes of material may beremoved or omitted from the mounting structure during fabrication ordesign (e.g., in the machining casting, molding, etc.) to provide, forexample, pockets, cavities, holes, spaces, voids, etc., for the purposeof weight reduction, as well as to impart improved mechanical stiffness,thermal stability, etc. For example, in the case of a cast or moldedstructure, voids, pockets, spaces, holes, cavities, etc., may bedesigned into the mounting structure to reduce the total mass (and cost)of the holographic drive head assembly, while simultaneously improvingstiffness of the mounting structure to minimize motion effects (e.g.,vibrations). Removal or omission of material from the mounting structureduring fabrication or design may also be used to minimize one or moreexternal dimensions of the structure, as well as to provide passivealignment means for mounted components and/or subassemblies.

The mounting structure may also be provided with preselected locationsfor mounting components and/or subassemblies of the holographic drivehead that require or may require optical alignment. By providingpreselected locations on the mounting structure for mounting thesecomponents and/or subassemblies, holographic drive head assemblies maybe formed, made, manufactured, etc., to have consistent, uniform, etc.,optical alignment of the various components and/or subassemblies withminimal or no need for extensive alignment of these components and/orsubassemblies after mounting. Providing preselected mounting locationsmay also provide improved mechanical and thermal stability as well asmore accurate placement tolerances for the mounted components and/orsubassemblies.

One or more passive alignment members may also be provided inconjunction with mounting some, most or all of these components and/orsubassemblies at these preselected locations. For example, the abilityto interchange holographic storage media between different holographicdata storage devices (e.g., to read data from a holographic medium ormedia in one device which was recorded in a different device) mayrequire precise alignment of the reference beam optical path (e.g., path260 of FIG. 2A) with respect to the data beam optical path (e.g., path262 of FIG. 2A) and the holographic medium (e.g., storage disk 238 ofFIG. 2A). Previously, this alignment procedure could be extremely timeconsuming and necessitate complicated hardware to attain the level ofadjustment and precision needed. Minimizing or eliminating the need toactively align the reference beam path to the data beam path may thus bedesirable from a manufacturing and performance standpoint. By using aunitized mounting structure in combination with preselected mountinglocations and passive alignment members, more precise, consistent,uniform, etc., manufacturing and assembly techniques for holographichead drive assemblies may be carried out which result in the desiredrelationship and optical alignment between the reference beam path (orportion thereof) and the data beam path (or part thereof) being achievedwith minimal or no active adjustment being required.

These passive alignment members may be separate from the mountingstructure, may be integral therewith, or may be a combination thereof.For example, the mounting structure may be provided with built-inpassive alignment members (e.g., passive alignment rails, guides,grooves, walls, surfaces, datums, features, etc., or combinationsthereof) to which the components and/or subassemblies of the holographicdrive head assembly that require or may require optical alignment arepositioned to, mounted on, attached to, etc. Some illustrativeembodiments of passive alignment members that may be used with, formedin, etc., the mounting structure to optically align the componentsand/or subassemblies of the holographic drive head assembly are shown inFIGS. 3-5. FIG. 3 illustrates one embodiment of a passive alignmentmember that may be separate from or be formed in (i.e., is integralwith) the mounting structure, which is indicated generally as 300. Asshown in FIG. 3, passive alignment member 300 is used to mount and aligna subassembly of components, for example, a lens subassembly 308comprising a pair of axially spaced apart optical lenses, indicated as316 and 320. Although not shown, lens subassembly 308 may include ahousing (e.g., a generally cylindrically-shaped housing) for integratinglens 316 and 320 into subassembly 308. A shown in FIG. 3, a generallyV-shaped groove 328 may be formed in the upper surface 336 of member300. Groove 328 comprises a pair of datum or reference surfaces 340 and344 joined or intersecting at an edge 348. Surfaces 340 and 344 mountlens subassembly 308 by engaging the outer circumferential surface 352of lens 316 and the outer circumferential surface 360 of lens 320.

The angle formed by the intersection of surfaces 340 and 344 at edge348, as well as the diameter of lenses 316 and 320, determine at whichpoint surfaces 352 and 360 engage surfaces 340 and 344. In fact,altering or changing the angle between surfaces 340 and 344 may be usedto vertically adjust how far down or up lenses 316 and 320 are seatedwithin groove 328. Similarly, altering or changing the diameter oflenses 316 and 320 may also be used to adjust how far down or up lenses316 and 320 are seated within groove 328. Because the angle betweensurfaces 340 and 344 is often constant or fixed, and because thediameters of 316 and 320 are also often constant or fixed, lenssubassembly 308 may be mounted at a constant or fixed vertical positionor depth within groove 328. This results in a known positioning of lenssubassembly 308 which is independent of the axial position ofsubassembly 308 along the length of the groove 328 and which may be usedto ensure optical alignment with respect to other components and/orsubassemblies of the holographic drive head assembly. For example, theaxial distance between lenses 316 and 320 may be adjusted without alsocausing a lateral shift between the lenses 316 and 320. Such adjustmentsmay be a common assembly step during the alignment of a holographicdrive head and/or one or more of the respective components and/orsubassemblies.

FIG. 4 illustrates another embodiment of a passive alignment memberindicated generally as 400 which comprises a pair of generallycylindrical laterally spaced apart guides or rails, indicated as 408 and412. Rails 408 and 412 are used to mount and align a subassembly ofcomponents, for example, a lens subassembly 418 (e.g., similar tosubassembly 308 of FIG. 3) comprising a pair of axially spaced apartoptical lenses, indicated as 426 and 430. As shown in FIG. 4, the outercircumferential surfaces 434 and 438 of, respectively, rails 408 and412, mount lens assembly 418 by engaging the outer circumferentialsurface 442 of lens 426 and the outer circumferential surface 446 oflens 430.

The respective diameters of rails 408 and 412, in combination with thedistance between surfaces 434 and 438, as indicated by double headedarrow 454, determine at which point surfaces 442 and 446 of lenses 426and 430 engage rails 408 and 412. If the diameters of rails 408 and 412are kept constant or fixed, the positioning of lens assembly 418 may beadjusted vertically up or down. In fact, if distance 454 is keptconstant or fixed for rails 408 and 412, lens subassembly 418 may bemounted on rails 408 and 412 at a constant or fixed vertical position.Like passive alignment member 300 of FIG. 3, providing a constant orfixed distance 454 results in a known positioning of lens assembly 418that may be used to ensure optical alignment with respect to othercomponents and/or subassemblies of the holographic drive head assembly.

FIG. 5 illustrates an embodiment comprising a pair of passive alignmentmembers which is used in combination with a pair of component orcomponent subassemblies, with the combination being indicated generallyas 500. As shown in FIG. 5, combination 500 comprises a pair ofcomponent or component subassemblies which may be in the form of areference beam lens component or component subassembly (e.g., scannerlens subassembly 250 of FIG. 2A), indicated generally as 508, and a databeam lens component or subassembly (e.g., storage lens 236 of FIG. 2A),indicated generally as 516. Lens component/assemblies 508 and 516 aremounted on or by a mounting member, indicated generally as 524. Mountingmember 524 may have formed therein a pair of passive alignment recesses528 and 532. Recess 528 has a bottom surface 534, and a pair of spacedapart generally parallel walls 538 and 542 extending upwardly frombottom surface 534. Recess 532 also has a bottom surface 550, and a pairof spaced apart generally parallel walls 554 and 558 extending upwardlyfrom bottom surface 550.

As shown in FIG. 5, recess 528 may be provided with a pair of generallycylindrical spaced apart guides or rails 570 and 574. Rail 570 engagesrecess wall 538, while rail 574 engages recess wall 542. Lenscomponent/subassembly 508 is mounted on rails 570 and 574. Recess 532may also be provided with a pair of generally cylindrical spaced apartguides or rails 580 and 584. Rail 580 engages recess wall 554, whilerail 584 engages recess wall 558.

Lens component/subassembly 508 may be mounted on rails 570 and 574,while lens subassembly 516 may be mounted on rails 580 and 584. Assumingthat the diameter of lens component/subassembly 508, as well as thediameter of rails 570 and 574, is kept constant or fixed, the point atwhich lens component/subassembly 508 engages rails 570 and 574 isdetermined by the distance between rails 570 and 574, as indicated bydouble headed arrow 588, which is affected or controlled by the width ofrecess 528 (between walls 538 and 542), as indicated by double headedarrow 592. In other words, by changing the width 592 of recess 528, thedistance 588 between rails 570 and 574 may also be adjusted orcontrolled, thereby adjusting the vertical positioning of lenssubassembly 508. Alternatively, the vertical positioning of lenscomponent/subassembly 508 may instead be adjusted by the insertion ofone or more spacers between either rail 570 and wall 538, or rail 574and wall 542, thereby decreasing the width 588 without modifying recess528. Also alternatively, the vertically positioning of lenscomponent/subassembly 508 may be adjusted by altering or changing thediameter of rails 570 and 574 (thus changing distance 588), thus keepingwidth 592 constant or fixed.

Similarly, assuming that the diameter of lens component/subassembly 516,as well as the diameter of rails 580 and 584, is kept constant or fixed,the point at which lens subassembly engages rails 580 and 584 isdetermined by the distance between rails 580 and 584, as indicated bydouble headed arrow 594, which is affected by the width of recess 532(between walls 554 and 558), as indicated by double headed arrow 596. Inother words, by changing the width 596, the distance 594 between rails580 and 584 which may be adjusted or controlled, thereby adjusting thevertical positioning of lens component/subassembly 508. Alternatively,the vertical positioning of lens component/subassembly 516 may insteadbe adjusted by the insertion of one or more spacers between either rail580 and wall 554, or rail 584 and wall 558, thereby decreasing the width594 without modifying recess 532. Also alternatively, the verticalpositioning of lens component/subassembly 516 may be adjusted byaltering or changing the diameter of rails 580 and 584 (thus changingdistance 594), while keeping width 596 constant or fixed.

In fact, the optical alignment between lens component/assembly 508 and516 may be adjusted or controlled by controlling the distance 594between rails 580 and 584, as well as controlling the distance 596between rails 580 and 584. In an embodiment of 500, the distances 5

be kept constant or fixed to ensure optical alignment between lenscomponent/subassembly 508 and 516, as well as with respect to othercomponents and/or subassemblies of the holographic drive head assembly.

Passive alignment members, for example, such as those illustrated inFIGS. 3-5 may provide a precise, consistent, uniform, etc., means forconstraining and optically aligning components and/or subassemblies ofthe holographic drive head assembly, for example, those componentsand/or subassemblies in reference beam path 260 and data beam path 262(see FIG. 2A). These passive alignment members may permit the mountingof, for example, optical components and/or assemblies, within preciseoptical tolerances (e.g., tolerances along the axis of the optical pathfrom one lens to another lens on the order of about 0.0005″) that may berequired in holographic data storage device, such as HMS 200. As alreadyalluded to with respect to the embodiment of FIG. 3, optical alignmentof the various components of the holographic drive head assembly may befurther improved through aggregation or combination of these componentsinto subassemblies where, for example, the optical inputs and outputs ofthese components are collimated light. Such unitization of thecomponents into subassemblies may also separate or isolate lateral andangular tolerance requirements of the optical components (e.g., lenses,etc.) from the mounting structure. Such subassemblies may also simplifythe axial alignment between two components by enabling “Z” translationindependent of radial position requirements or error issues (i.e., theaxial spacing between the various components and/or subassemblies).

The manufacture and optical alignment of the components and/orsubassemblies of the holographic drive head assembly mounted on themounting structure may be further simplified by providing that thepreselected mounting locations are oriented such that one or more (e.g.,some, most or all) pairs of adjacent or sequential components orsubassemblies are not coaxial, i.e., are not linearly aligned. Toutilize more efficient manufacturing and assembly principles, eachsubassembly of components may be independently assembled and aligned andthen mounted on the mounting structure with other subassemblies and/orcomponents of the holographic drive head assembly. To optically linkthese separate subassemblies on the mounting structure, optical pathbending or altering elements (e.g., reflectors, mirrors, prisms,mirrored prisms) may be positioned or placed between each pair ofadjacent or sequential subassemblies and/or components that are notcoaxial so that input and output optical radial alignment tolerances maybe decoupled from the positioning and mounting of the subassembliesand/or components on the mounting structure. In addition, if thesubassembly and/or component uses a passive alignment feature, such asV-groove 328 shown in FIG. 3, then any necessary focus adjustment, i.e.,an adjustment in the axial spacing between the subassemblies and/orcomponents, may be performed independent of the radial optical alignmenttolerances. This decoupling may also enable a more rigid mounting ofthese subassemblies and/or components, and thus potentially greaterresistance or immunity against motion effects (e.g., vibrations, shocks,impacts, etc.). This decoupling may also enable the use of “pick andplace” techniques to position and mount the optical path bending oraltering elements. Paired optical path bending or altering elements mayalso be used to align separate optical components and/or assemblies tocreate planar or compact geometries. In some embodiments, the use ofoptical path bending or altering elements, in combination withpreselected mounting locations and passive alignment members, may enablemultiple optical components and/or assemblies to be mounted within tightoptical axis tolerances because these components and/or assemblies maybe placed or positioned on surfaces that are nearly or exactly coplanar.

FIGS. 6-12 illustrate an embodiment of a mounting structure withpreselected locations for mounting holographic drive head componentsand/or subassemblies, along with passive alignment members integralwith, as well as separate from the mounting structure. Although notshown in FIGS. 6-12, the mounting structure, indicated generally as 600,may use optical path bending or altering elements to optically linkseparate subassemblies and/or components of the holographic drive headassembly (especially where adjacent pairs of subassemblies and/orcomponents are not coaxial) into a fairly compact and optically alignedsystem that may require minimal or no active alignment of the respectiveoptical paths, e.g., reference beam path 260 and data beam path 262 ofFIG. 2A.

Referring to FIG. 6, mounting structure 600 has an upper or top side,indicated generally as 602, and a forward half or section, indicatedgenerally as 604, and a rearward half or section, indicated generally as606. Top side 602 may be provided with a plurality of preselectedmounting locations. For example, as shown in FIG. 6, top side 602 may beprovided with a preselected SLM/PBS subassembly mounting location,indicated generally as 608, that is positioned in rearward section 606closest to lateral side 610, a preselected relay lens subassemblymounting location, indicated generally as 612, positioned near oradjacent to SLM/PBS subassembly mounting location 608 and along lateralside 610 in rearward section 606, a preselected mounting block mountinglocation, indicated generally as 616, positioned in rearward section 606near or adjacent relay lens subassembly mounting location 612, and apreselected scanner lens subassembly mounting location, indicatedgenerally as 620, positioned in rearward section 606 proximate lateralside 622 and near or adjacent mounting block location 616.

As shown in FIG. 6, SLM/PBS subassembly mounting location 608 comprisesa horizontal mounting or referencing surface 624 and a vertical mountingor referencing surface 628 which is oriented generally orthogonal tosurface 624. Relay lens subassembly mounting location 612 comprises afirst horizontal mounting or referencing surface 632, a secondhorizontal mounting or referencing surface 634 which is orientedgenerally coplanar with surface 632, and a vertical mounting orreferencing surface 636 which is oriented generally orthogonal tosurfaces 632 and 634. Mounting block location 616 comprises a horizontalmounting or referencing surface 640, a first vertical mounting orreferencing surface 642 which is oriented generally orthogonal tosurface 640, and a second vertical mounting or referencing surface 644which is oriented generally orthogonal to both of surfaces 640 and 642.Scanner lens assembly mounting location 620 is shown in FIG. 6 ascomprising horizontal mounting or referencing surface 648 which isgenerally coplanar with surface 640.

As further shown in FIG. 6, top side 602 may be provided with agenerally U-shaped recess 652 in rearward section 606 and proximatelateral side 622 which may be used to mount, for example, galvo 252 (seeFIG. 2A). Recess 652 may have formed therein a hole 656 through which,for example, reference beam path 260 may extend from mirror 293 to galvo252 (see FIG. 2A). Forward section 604 may be provided with acompartment or bay 664 for mounting, for example, laser 204 (see FIG.2A), which opens outwardly through side 610. At one end of bay 664 is aU-shaped recess 672 provided, for example, for mirror 290 (see FIG. 2A).The arrow 676 indicates the holographic medium loader area where theholographic medium is laterally transported to and from the holographicdrive head assembly by a loader (not shown). Four mounting locations orpads for the loader (not shown) are indicated by arrows 678. Anoval-shaped aperture 680 is formed in forward section 604 of structure600 to receive the drive spindle of the loader (not shown). Arrows 688and 696 indicate weight reduction and/or stability pockets that may beformed in structure 600.

Referring to FIG. 7, structure 600 has a bottom or underside, indicatedgenerally as 702. Underside 702 is shown in FIG. 7 as also beingprovided with a plurality of preselected mounting locations. Forexample, underside 702 may be provided with a preselected relay lenssubassembly mounting location, indicated generally as 704, which ispositioned at the end of rearward section 606, and a preselected dataexpander subassembly mounting location, indicated generally as 708,which is positioned in rearward section 606 and proximate side 610.Relay lens subassembly mounting location 704 comprises a firsthorizontal mounting or referencing surface 712, a second horizontalmounting or referencing surface 714 which is oriented generally coplanarwith surface 712, a first vertical mounting or referencing surface 716which is oriented generally orthogonal to surfaces 712 and 714, and asecond vertical mounting or referencing surface 720 which is orientedgenerally coplanar with surface 716, and generally orthogonal tosurfaces 712 and 714. Vertical surfaces 716 and 720 are laterally spacedapart and are positioned on downwardly extending mounting wall 722. Dataexpander subassembly mounting location 708 comprises a first horizontalmounting or referencing surface 726, a second horizontal mounting orreferencing surface 730 which is oriented generally coplanar withsurface 726, a first vertical mounting or referencing surface 734 whichis oriented generally orthogonal to surfaces 726 and 730, a secondvertical mounting or referencing surface 738 which is laterally spacedapart from but which is oriented generally coplanar with surface 734 andwhich is oriented generally orthogonal to surfaces 726 and 730, and athird vertical mounting or referencing surface 742 which is orientedgenerally orthogonal to surfaces 726, 730, 734 and 738. Verticalsurfaces 734 and 738 are laterally spaced apart and are positioned ondownwardly extending mounting wall 740 proximate side 622.

As further shown in FIG. 7, underside 702 may be provided with cablerecesses or slots, some of which are generally indicated as 744. Arrow756 indicates a mounting area for mounting a component, for example, aprism (not shown). Mounting area 756 may include a triangular array ofthree protruding mounting bosses, indicated as 760, on which thecomponent may be mounted. Also shown in FIG. 7 is a mounting area,indicated as 764, in the form of a slot positioned between surfaces 712and 726 for mounting or locating, for example, phasemask 222 (see FIG.2A), and a mounting area or surface, indicated as 768, for mounting orlocating, for example, variable optical divider 214 (see FIG. 2A). Alsoshown in FIG. 7 is a compartment or bay 772 along lateral side 622 inwhich, for example, main expander assembly 212 and apodizer 210 (seeFIG. 2A) may be mounted or located.

FIGS. 8-9 illustrate an embodiment of a mounting block, indicatedgenerally as 800, that may be used with structure 600 as a passivealignment member. Mounting block 800 comprises a larger mountingsection, indicated generally as 804, and an adjacent or adjoiningsmaller mounting section, indicated generally as 808. Larger mountingsection 804 and smaller mounting section 808 together have or define apreselected storage lens subassembly mounting location, indicatedgenerally as 812, and a preselected scanner lens subassembly mountinglocation, indicated generally as 816. Storage lens subassembly mountinglocation 812 comprises a vertical mounting or referencing surface 820positioned on larger mounting section 804, and an adjacent or adjoiningslanted mounting or referencing surface 824 positioned on smallermounting section 808 which is oriented generally transversely relativeto surface 820 and which shares a common slanted edge 826. Scanner lenssubassembly mounting location 816 comprises a first vertical mounting orreferencing surface 828 positioned on smaller mounting section 808, asecond vertical mounting or referencing surface 830 positioned on largermounting section 804 which is oriented generally orthogonal to surface828, and a slanted mounting or referencing surface 834 which shares acommon slanted edge 836 with adjacent or adjoining surface 828, which isoriented generally transversely relative to surfaces 828 and 830, andwhich shares a common horizontal edge 840 with slanted surface 824. Aclamping surface 844 (e.g., for the storage lens subassembly) may bepositioned on larger mounting section 804 and is oriented generallytransversely relative to adjacent or adjoining surface 820, shares acommon slanted edge 846 with surface 820, and is parallel to surface824. A first generally rectangular or square-shaped vertical blocklocating or referencing surface 848 is also positioned on large mountingsection 804 below adjacent or adjoining surface 844, and shares commonhorizontal edge 850 with surface 844 and a common vertical edge 852 withsurface 820. Also indicated by FIG. 8 is a second vertical blocklocating or referencing surface 854 which is also positioned on largemounting section 804, and which is oriented generally orthogonal toadjacent or adjoining surfaces 848 and 830, and shares a common verticaledge 856 with surface 848. Mounting block 800 also has a bottom surface,indicated generally as 860, for mounting block 800 on the top side 602of structure 600, as described below.

FIGS. 10-12 illustrate how the various components and/or subassembliesof the holographic drive head may be mounted on structure 600. Referringto FIGS. 10-11, SLM/PBS subassembly 226/258 (see FIG. 2A) may be mountedat mounting location 608, with surfaces 624 and 628 referencing andorienting subassembly 226/258 in the proper optical alignment. Relaylens subassembly 232 (see FIG. 2A) may be mounted at mounting location612, with surfaces 632, 634 and 636 referencing and orientingsubassembly 232 in the proper optical alignment. Mounting block 800 ismounted by bottom surface 860 on reference surface 640 at mountinglocation 616, with mounting block locating surfaces 848 and 854 adjacentor abutting respective vertical surfaces 642 and 644 of mountinglocation 616 so that mounting block 800 and its respective mountinglocations 812 and 816, along with respective reference surfaces 820,824, 828, 830, and 834, are properly oriented. Storage lens subassembly236 (see FIG. 2A) may be mounted at mounting location 808 with surfaces820 and 824 referencing and orienting subassembly 236 in the properoptical alignment. Scanner lens subassembly 250 (see FIG. 2A) may bemounted at mounting locations 620 and 816 with surfaces 648, 828, 830,and 834 referencing and orienting subassembly 250 in the proper opticalalignment, including proper optical alignment with respect tosubassembly 236. Slanted surfaces 824 and 834 generally control theangle at which scanner lens subassembly 250 and storage lens subassembly236 are oriented with respect to each other so that reference beam 280transmitted from scanner lens subassembly 250 and data beam 282transmitted from storage lens subassembly 236 may interfere to formholograms. Alternatively, and in place of mounting block 800, storagelens subassembly 236 and scanner lens subassembly 250 may be mounted onstructure 600 using mounting 524 and associated pairs of rails 570/574and 580/584 shown in FIG. 5.

Referring to FIG. 12, relay lens subassembly 224 (see FIG. 2A) may bemounted at mounting location 704, with surfaces 712, 714, 716 and 720referencing and orienting subassembly 224 in the proper opticalalignment. Data expander subassembly 220 (see FIG. 2A) may be mounted atmounting location 708, with surfaces 726, 730, 734, 736, 738, and 742referencing and orienting subassembly 220 in the proper opticalalignment. As further shown in FIGS. 10-12, adjacent pairs ofsubassemblies may be oriented so as to be non-coaxial, for exampleadjacent subassembly pairs 236 and 232, adjacent subassembly pairs226/258 and 224, and adjacent subassembly pairs 224 and 220. With regardto subassembly pair 236 and 232 pair, a pair of adjustable mirrors maybe positioned between subassembly 236 and 232.

Description of Spatial Light Modulator, Beam Splitter and Detector Array(e.g., Camera) Subassembly Alignment and Physicallmaged Aperture forSpatial Light Modulator

Embodiments of the present invention also relate to an alignedsubassembly comprising a spatial light modulator (SLM), a (e.g.,polarizing) beam splitter and a detector array (e.g., camera).Holographic data storage systems, such as HMS 200 of FIG. 2A, mayrequire the precise alignment of the detector array (e.g., camera) inmultiple directions of freedom to attain the necessary optical andmechanical relationship with the SLM. Optically and mechanicallyaligning an SLM and a detector array (e.g., camera) with a commonadjacent component, such as a polarizing beam splitter (PBS),independent of the alignment of the remaining holographic drive headcomponents and/or subassemblies may be difficult to achieve. Thisalignment procedure may be time consuming and necessitate complicatedhardware to attain the level of adjustment and alignment needed. Inaddition, eliminating the need to actively align the detector array(e.g., camera) may also be desirable. See commonly assigned U.S. PatentApplication 2005/0286388 (Ayres et al.), published Dec. 29, 2005.

In embodiments of the present invention, the aligned relationship (e.g.,mechanical and optical) between the SLM and the detector array (e.g.,camera), as well as a common adjacent beam splitter (e.g., PBS), may becontrolled with minimal or no need for active adjustment. For example,the SLM may be attached to a mount that may precisely control therelationship between the SLM and the PBS, or an intermediary mountingcomponent between the SLM and the PBS. Mounting surfaces may be commonlycontrolled to within 0.0005″ to maintain the proper relationships. Thismay be achieved, for example, by inserting microspheres into a bondingagent used to join the SLM and PBS together as a partial subassembly.This partial SLM/PBS subassembly may then be attached to the detectorarray (e.g., camera), for example, by either attaching the cameradirectly to the PBS or by using an intermediate mounting component(s)(e.g., mounting blocks) between the PBS and the camera. This unitizedSLM/PBS/camera subassembly may provide the SLM and camera as a singleunit which is aligned and ready to be integrated into the holographicdrive head assembly. In addition, if the SLM and/or camera shouldrequire replacement because of malfunction, failure, damage, etc., itmay be possible to remove the old SLM/PBS/camera subassembly and replaceit with a similar (new) SLM/PBS/camera subassembly with minimal or noadjustment or alignment required.

An embodiment of the SLM/PBS/camera subassembly according to the presentinvention is illustrated in FIGS. 13-14. Referring to FIG. 13, a partialsubassembly, indicated generally as 900, comprising the SLM, indicatedgenerally as 908, and a generally cube-shaped PBS, indicated generallyas 916, is shown. A passive SLM mounting structure in the form of apedestal, indicated generally as 924, may be provided for housing SLM908, and may also comprise the circuit or circuit board 925 for poweringor activating SLM 908. SLM mounting pedestal 924 may be provided with abase, indicated generally as 932, and a plurality of mounting members inthe form of four spaced apart mounts arranged in a generallysquare-shaped configuration and extending outwardly from base 932, threeof which are shown in FIG. 13 and are indicated as 936, 940 and 944. Asshown in FIG. 13, one face of PBS 916, indicated as 948, may be mounted,attached, secured, etc., to mounts 936, 940 and 944 (for example,through the use of a bonding agent or adhesive), so that adjacent andopposing face 948 of PBS 916 is joined to SLM 908 to form partialsubassembly 900 comprising SLM 908 and PBS 916.

Referring to FIG. 14, partial SLMIPBS subassembly 900 is attached,secured, etc., to the camera subassembly, indicated generally as 956, toform the completed SLMIPBS/camera subassembly, indicated generally as960. Camera subassembly 956 comprises a camera 964, a circuit board 970to which camera 964 is attached, mounted, secured, etc., for powering oractivating the camera 964, and a means for associating camerasubassembly 956 to partial SLMIPBS subassembly 900 in the form of acamera PCB stiffening frame 974 attached to the circuit board 970. Apair of passive mounting blocks may also be provided, one of which,indicated as 978, is attached (e.g., by adhesive or bonding agent) toface 980 of PBS 916, the other of which, indicated as 982, is attached(e.g., by adhesive or bonding agent) to the opposite face 984 of PBS916. Mounting blocks 978 and 982, together, may be used to mount,attach, secure, etc., PBS 916 of SLM/PBS subassembly 900 to camera frame974 of camera subassembly 956 by means of, for example, fasteners, etc.(not shown) to form unitized SLM/PBS/camera subassembly 960. Face 990 ofPBS 916 (which is perpendicular to face 948 positioned adjacent to andopposing SLM 908, as well as opposing faces 980 and 984) is positionedadjacent to and opposing camera 964 such that SLM 908 and camera 964 (aswell as camera subassembly 956) are optically and mechanically alignedwith respect to PBS 918 and with respect to each other. Alternatively,face 990 of PBS 916 of SLM/PBS subassembly 900 may be directly mounted,attached secured, etc., to camera 964 (e.g., through the use of anadhesive or bonding agent) without the use of mounting blocks 978 and982.

The SLM/PBS/camera subassembly 960 may be used, for example, in themethod and holographic data storage system described in commonlyassigned U.S. Patent Application 11/069,007 (Ayres et al.), filed Feb.28, 2005, the entire disclosure of which is incorporated by reference,to avoid the need for actively controlling the alignment of SLM 908, PBS916 and especially camera 964 through the use of, for example,microcontrollers, such as 117 and 129. The SLM/PBS/camera subassembly960 may also be used, for example, in place of separate SLM 226, camera228 and PBS 258 of HMS 200.

Embodiments of the present invention further relate to a spatial lightmodulator (SLM) provided with: (1) a physical aperture having highabsorption which is positioned over the photoactive area of the SLM; or(2) an imaged aperture projected onto the photoactive area, wherein thephysical or imaged aperture reduces reflections in the photoactive areawhich may, for example, cause degradation of the signal to noise ratio(SNR) of recorded holographic data. Reflections from bonding wires,pads, unused border pixels, etc., may result in unwanted noise signalsrecorded or written into the recorded holographic data which may degradethe overall SNR of recovered data. Wire bonds and pads may present aconstant noise signal by creating off-angle stray light. Unused borderpixels may directly contribute to the reduction of the dynamic range ofthe holographic storage medium because light coming from these borderpixels may not be used for image recreation or reconstruction. Theproblem of illuminated border pixels which do not carry data may beunavoidable when the SLM is based on ferroelectric liquid crystals whichmust be operated in a DC balanced mode, i.e., when averaged over sometime scale (typically from 100 to 1000 milliseconds), each SLM pixel isdriven on (bright) and driven off (dark) an equal amount. If this DCbalance refresh rate does not coincide or cannot exactly match therecording exposure rate, which may allow the border pixels to alternatebetween off (dark) during a recording exposure and on (bright) betweenrecording exposures when no light reaches the SLM, then during asignificant number of recorded pages, the border pixels may be in an on(bright) state during a significant portion of the recorded holographicdata (e.g., as much as half of the recorded pages). This may result inincreased light scatter from the border pixels, thereby degrading thesignal to noise performance of the system. Also, the border pixels mayadd coherently, thus creating an unwanted “Direct Current (DC) hot spot”in the holographic medium. The irradiance (intensity) of this DC hotspot may be as much as six orders of magnitude higher than the areasurrounding this DC hot spot.

Positioning a physical aperture having high absorption (e.g., providedby a high absorption coating) over (or imaging an imaged aperture onto)the photoactive area of the SLM may reduce, diminish, lessen, etc., theunwanted light from the recording operation. An embodiment of asubassembly comprising the combination of a physical aperture associatedwith the SLM is illustrated in FIG. 15 and is indicated generally as1000. As shown in FIG. 15, subassembly 1000 comprises an SLMsubassembly, indicated generally as 1008, and a physical apertureelement, indicated generally as 1016. SLM subassembly 1008 comprises anSLM circuit board, indicated generally as 1024, and an SLM, indicatedgenerally as 1032, which is powered or activated by board 1024. SLM 1032has a photoactive area 1042 (shown in FIG. 15 as having a rectangular orsquare shape but which may have other shapes) which defines theoptically active portion of SLM 1032 and which often has a cover made,for example, from glass.

Aperture element 1016 may comprise an aperture frame 1050 (shown in FIG.15 as having a generally rectangular or square shape but which may haveother shapes, often depending on the shape of photoactive area 1042)having a physical aperture 1058 formed therein and covered with, forexample, glass having thereon a high absorptive material, a thin sheetof steel coated with black oxide, etc. Aperture element 1016 may berelatively thin, for example, having a thickness of from about 0.005 toabout 0.1 inches, more typically from about 0.025 to about 0.075 inches.The shape of aperture 1058 may be of the same general shape asphotoactive area 1042 (e.g., a generally rectangular shaped, a generallysquare shape, etc.), or may have a different shape (e.g., a generallycircular shape, including a generally circular shape with parallelchords defining parallel straight edges and alternating arcs havingequal radii and coincident centers of curvature, a generally octagonalshape, including a generally octagonal shape comprising four alternatingedges in the form of arcs having equal radii and coincident centers ofcurvature, etc.) depending on the imaging capabilities of the opticalsystem (e.g., relay lens 224) and the desired size and shape of the datapage to be recorded. The dimensions of aperture 1058, i.e., in lengthand width, as indicated respectively by double headed arrows 1062 and1066, may be such that aperture 1058 may have exactly the samedimensions as photoactive area 1042, i.e., in length and width, asindicated respectively by double headed arrows 1070 and 1074, up tobeing slightly larger in dimensions (e.g., oversized by a few pixels).As indicated by broken lines 1078, aperture element 1016 may be placedor positioned over or on SLM 1032 so that aperture 1058 is over andaligned with photoactive-area 1042. Frame 1050 of aperture element 1016may be secured, attached, mounted, etc., to SLM 1032 by using low forcemechanical mount, an adhesive, etc. Once aperture element 1016 issecured, attached, mounted, etc., to SLM 1032, the resultant combination1000 may only reflect light from pixels within the exposed area byaperture 1058, thus reducing, diminishing, lessening, eliminating, etc.,stray light, border light, etc., that does not contribute to datadecoding by SLM 1032.

Description of Alignment and Replacement of Primary Laser

The embodiments of the present invention also relate to alignment of theprimary laser (e.g., laser 204 of FIG. 2A) used in a holographic drivehead assembly when the primary laser is newly installed in the systemeither during initial manufacture of the system, or after the primarylaser is replaced with a new laser. Because of laser failure,malfunction, damage, etc., it may be necessary to replace the primarylaser in a holographic drive head assembly. Because the primary laser isnormally the first optical component in a linked chain of opticalcomponents and/or subassemblies, replacing the original laser with a newlaser may require the realignment of all of the components orsubassemblies in the holographic drive head assembly. Such alignment maybe a very time consuming task.

Alignment of the original or replacement primary laser with respect tothe optical path (e.g., optical path 112 of FIGS. 1 and 2A) may beachieved, and in the case of a replacement laser, without the need toalign other components and/or subassemblies that come after the laser inthe holographic drive head assembly, by using one or more embodimentsaccording to the present invention, In one embodiment for opticallyaligning the laser with respect to the optical path, a pair ofindependent optical path bending or altering elements (e.g., reflectors,mirrors, prisms, mirrored prisms) may be inserted or positioned in theoptical path after the laser and before the next optical component orsubassembly in the optical path (e.g., main expander assembly 212 ofFIG. 2A). Use of the pair of optical path bending or altering elementsenables the laser beam to be oriented so that any misalignment may becorrected for.

In another embodiment, a spatial filter having associated therewith apinhole through which light generated by the laser may pass may be used,wherein the amount of light passing through the pinhole is used todetermine the optical alignment of the laser with respect to the opticalpath. Alignment of an original or replacement laser in a holographicdrive head assembly may be further simplified by using the feedback froma spatial filter pinhole (e.g., the combination of lens 274 and pinhole206 shown in FIG. 2A). The spatial filter pinhole may require that thelaser input be within about 10 arc seconds of the replaced (original)laser position to allow light to pass through. Such alignment tolerancesmay be achieved through optimization of the amount of light passingthrough the pinhole by adjusting two optical components or subassembliesimmediately after the laser in the optical path. By performing thisoptimization, the pointing location of the replacement (new) laser maybe very close to the replaced (original) laser that the componentsand/or subassemblies of the system were initially aligned to. Inparticular, the spatial filter pinhole may be used to characterize thequality of the alignment.

In another embodiment which may be used separately or in conjunctionwith a spatial filter pinhole, one or more alignment apertures may beinserted or positioned in the optical path to facilitate alignment ofthe original laser with respect to the optical path, or a replacementlaser relative to the existing alignment of the optical componentsand/or subassemblies. These alignment apertures may define an opticalpath originally created by the replaced (original) laser. Potentialplacement locations for two alignment apertures which are axially spacedalong the opticial path in HMS system 200 (see FIG. 2A) is illustratedby FIG. 16 which shows a portion of system 200 of FIG. 2A. In FIG. 16,one alignment aperture, indicated as 1104, may be positioned afterapodizer 210 and before variable optical divider 214. The otheralignment aperture, indicated as 1108, may be positioned after variableoptical divider 214, e.g., in data beam path 262 after data beamexpander 220 and before phasemask 222. Other placement locations foralignment apertures 1104 and 1108 may also be used, as well as more thantwo alignment apertures, in aligning a new or replacement laser. Thealignment apertures 1104 and 1104 may also be removed from and thenreinserted into the optical path as needed.

By determining how much light gets through the two alignment apertures,for example, alignment apertures 1104 and 1108, and by maximizing thisthroughput, positioning and alignment of the replaced (original) lasermay be duplicated. In an alternative embodiment, if the two alignmentapertures are appropriately sized, a detector array (e.g., a camera) maybe positioned after the alignment apertures used to evaluate therelative alignment of the zeroth-order (or greater) of the diffractionpattern of the first alignment aperture (often referred to as “the Airydisc”) within the diffraction pattern created by the second alignmentaperture. In practice, this alternative embodiment is similar toaligning a spot or bullseye pattern (i.e, the diffraction pattern of thefirst alignment aperture) within the center of a second bullseye pattern(i.e, the diffraction pattern of the second alignment aperture), and maybe particulary sensitive and capable of a micron-level positionalresolution and an angular resolution limited by the effective distancebetween the two alignment apertures.

This alternative laser alignment system is illustrated schematically inFIG. 17, and is indicated generally as 1200. As shown in FIG. 17,alignment system 1200 comprises a first generally circular alignmentelement 1204, a second generally circular alignment element 1208 whichis axially spaced from first alignment element 1204 along the opticalpath and which may be generally coaxial or aligned with first alignmentelement 1204, and a detector such as camera 1212 which is axially spacedfrom second alignment element 1208 along the optical path and whichgenerally aligned with second alignment element 1208. First alignmentelement 1204 has a generally centered first alignment aperture 1216,while second alignment element 1208 has a generally centered secondalignment aperture 1220. First alignment aperture 1216 and secondalignment aperture 1220 are generally aligned and coaxial with eachother along the optical path. In fact, first alignment element 1204,second alignment element 1208 and detector 1212 may comprise a unitaryassembly, which may also be removed from and then reinserted into theoptical path as needed.

In operation, a collimated input beam, indicated generally as 1228,generated by the laser strikes surface 1232 of first alignment element1204. Only a portion of the light, indicated generally as 1234, exitsthrough and is diffracted by first alignment aperture 1216. A first setof concentric diffractions rings, indicated generally as 1238, are thusformed on surface 1240 of second alignment element 1208, comprising atleast an outer second-order diffraction ring, which is indicatedgenerally as 1244, and an inner first order diffraction ring, which isindicated generally as 1246 being a. The diameter of second alignmentaperture 1220 is sized so that the portion of light, indicated as 1250,comprising at least a portion of the first order diffraction ring 1246and the light forming zeroth-order diffraction ring 1266 is transmittedthrough the second alignment aperture 1220 of alignment element 1208.Light 1250 exiting through second alignment aperture 1220 also divergesand forms second set of concentric diffractions rings, indicatedgenerally as 1254, on reaching surface 1256 of camera 1212, whichcomprises at least an outer second-order diffraction ring, which isindicated generally as 1258, a middle first-order diffraction ring,which is indicated generally as 1262, and an inner the zeroth-orderdiffraction ring, which is indicated generally as 1266. The alignment ofat least the zeroth-order diffraction ring 1266 with respect to thecenter point, indicated as 1270, of the second set of diffraction rings1254 on surface 1256 of camera 1212 is used to determine the alignmentof the laser generating input beam 1228.

In using alignment system 1200, it may be necessary to temporarilyremove one or more components and/or subassemblies (e.g., data beamexpander 220 of FIG. 2A) from the optical path of input beam 1228 inorder properly position system 1200 while aligning the replacement (new)laser. In other words, alignment elements 1204 and 1208, as well ascamera 1212, may be removable in such a way that the positioning ofelements 1204 and 1208 is repeatable when removed and repositioned, andthus may be positioned in the optical path only when performing a laseralignment. Alternatively, input beam 1228 or a portion thereof may betemporarily diverted from the optical path of the holographic drive headassembly so that beam 1228 (or a portion thereof) enters alignmentsystem 1200.

In another embodiment, a portion of the light generated by the laser maybe diverted or split off to provide a monitoring beam, with themonitoring beam being used to determine whether the laser is opticallyaligned or pointing correctly. The primary laser (e.g., laser 204 ofFIG. 2A) of a holographic drive head assembly may be susceptible topointing errors. Changes in ambient temperature, laser wavelength, lasercurrent, etc., may cause such laser pointing errors which may result inthe holographic memory system not working properly. Accordingly, it maybe necessary to monitor the pointing of the laser, as well as tomaintain the pointing of the laser in the proper position.

In monitoring laser pointing, a beam splitter, such as, for example, awindow prism, or other pellicle type optical component, may be used tosplit off a portion of the laser beam to provide a monitoring beam. Thismonitoring beam may then be analyzed for any pointing discrepancies,errors, etc. The device for analyzing such pointing discrepancies,errors, etc., may be a photodetector, a bi-cell detector, a quad-celldetector, etc. Alternatively, a detector used in the holographic memorysystem, for example, camera 228 of FIG. 2A, may be used to monitor thelaser pointing.

Monitoring of laser pointing may also be used in conjuction orcombination with other components (e.g., electomechanical orelectro-optical actuators) to actively compensate or adjust for anylaser pointing errors that may be caused by temperature, wavelength,laser current, etc. For example, one or more path altering or bendingcomponents (e.g., mirrors, prisms, etc.) located in the optical pathafter the laser but before the spatial filter (e.g., lens 274 of FIG.2A) may be controlled, for example, by a servo to maintain the correctlaser pointing. A servo loop may be used to optimize laser pointingwhenever the holographic drive head assembly is not recording or readinga hologram so as to avoid affecting data transfer rates.

The embodiments of the present invention also relate to a new lightsource subassembly which may be used in a holographic drive headassembly. The variability of beam dimensions generated by a laser usedin a holographic drive head assembly may be a problem. The output beamsof many lasers used for such assemblies may have spatial intensityvariations or “noise” which is undesirable because it may lead directlyto a decreased signal to noise ratio (SNR) in the reconstructed data.Variability of beam dimensions may be especially significant when alaser diode is used as the light source because laser diodes may haveespecially larger variations in their beam properties, particularly theelliptical beam divergence angles. Beam uniformity may be required ifuniform illumination of any data page is needed, such as may be imposedon the data beam by a spatial light modulator (SLM). It may therefore benecessary to spatially filter such laser beams in order to improve beamuniformity.

One method of spatially filtering a laser beam involves focusing thelaser bean so as to pass through a small pinhole (e.g., pinhole 206 ofFIG. 2A), which may remove high spatial frequencies which may comprisethe unwanted noise. For a pinhole spatial filter such as 206, the sizeof the pinhole may be very small, e.g., in the range from 5 to 25microns, which may require precise alignment and robust positionalstability over time and over a range of operating temperatures, andduring or after the occurrence of motion effects (e.g., vibrations,shocks, impacts, etc.). Such alignment stability may be difficult toachieve.

As previously alluded to, replacement of a laser may require lengthy andtime-consuming procedures to ensure that the beam generated by thereplacement laser is aligned angularly and translationally correct, andif the generated beam size varies from unit to unit, to set anynecessary magnifications in the system, and to check the resultingsystem wavefront. It may also be necessary to optically isolate thelaser from any strong optical reflections which may cause instability inthe laser power and spectral mode (e.g., wavelength). The problem ofvariability of beam dimensions previously alluded to may also beespecially significant when a laser diode is used as the light sourcebecause laser diodes have an especially large variation in their beamproperties, particularly with regard to the elliptical beam divergenceangles.

This problem may be solved by using a light source subassemblycomprising a laser, a fiber coupling (focus) lens connected to thelaser; and an optical fiber connected to the fiber coupling lens andhaving a fiber connector ready output end. The optical fiber may be asingle-mode optical fiber and may also be a polarization-maintainingoptical fiber. The fiber connector ready output end may be anangled-type PC (APC) type of fiber connector. For shorter visiblewavelengths, the fiber core may comprise a material which does notsuffer unwanted absorption or other nonlinear loss mechanisms, such asstimulated brillouin scattering, especially when optical powers andhence fiber power densities are high for the holographic memory system(e.g., >50 mW laser power).

Use of a light source subassembly comprising a laser, a fiber coupling(focus) lens; and an optical fiber having a fiber connector ready outputend provides several advantages. The output beam divergence angle of anoptical fiber is determined by the numerical aperture of the light beamfocused on the input end of the optical fiber, as well as limited by thefiber's properties (core and cladding dimensions and materials). Thislight source subassembly may be designed and adjusted so that the outputof the optical fiber has consistent laser beam parameters, and thereforeholographic drive heads using this light source subassembly may beidentical, thus requiring less time to assemble and align compared tosystems requiring variable magnification inside the holographic drivehead.

In addition, a single-mode optical fiber only allows propagation of thefundamental mode of the fiber. High spatial frequency noise in typicallaser outputs represent higher order fiber modes which may not propagatein a single-mode fiber, potentially resulting in a pure spatiallyfiltered (Gaussian) beam being emitted from the fiber. The optical fiberthus performs the same beam filtering effect as a spatial filter but, bytaking advantage of fiber coupling techniques with high mechanicalstability, mechanical stability problems of free-space lenses and apinhole may be avoided.

Using an optical fiber having a fiber connector ready output end alsoenables extremely rapid and simple replacement of a laser in aholographic drive head. Such fiber connectors may have very preciserepeatability such that when the new light source subassembly isinstalled and fiber connector connected to the remainder of theholographic drive head assembly, no further alignment may be necessary.Use of angled fiber connectors (such as the APC-type), or angled fiberfaces, may ensure that very little light reflects back to the laser. Atthe fiber input end, the reflection from the fiber face may be angledsufficiently such that an insubstantial amount of power is collected bythe focusing lens and hence is unavailable to cause instability in thelaser. Also, internal reflections from the ouptut fiber face may bebeyond the collection NA of the fiber, may not achieve total internalreflection, and therefore may not significantly propagate back towardthe input end of the fiber. Careful design of the rest of the opticalsystem, for example, tilting all transmissive planar optics (e.g. cubepolarizing beam splitters, internal turning prisms, etc.) so thatincident light is not retroreflected back towards the laser may ensurethat sufficiently low power levels couple back into the fiber which maypotentially interfere with the stability of the laser.

An embodiment of this laser light source subassembly is illustrated inFIG. 18, and is generally referred to as 1300. Subassembly 1300comprises a laser source, indicated generally as 1304, which may be aconventional laser or a laser diode. Subassembly 1300 further comprisesa fiber optics coupling lens, indicated generally as 1312 which isconnected to laser source 1304. Subassembly 1300 further comprises anoptical fiber (e.g., a single-mode optical fiber), indicated generallyas 1318, which has an input end 1320 which is connected to fiber opticscoupling lens 1312. Optical fiber 1318 further comprises a fiberconnector ready output end 1322.

FIG. 19 is a schematic illustration of an embodiment of a fiber opticcoupled laser light source subsystem, which is generally referred to as1400, and which may be used in, for example, HMS system 200 illustratedin FIG. 2A, and which is generally referred to in FIG. 19 as 1402. Asshown in FIG. 19, subsystem 1400 comprises as laser source, indicatedgenerally as 1404, which generates a light beam, indicated as 1406,which is then transmitted to the beam conditioning optics, indicatedgenerally as 1408, of subsystem 1400. The conditioned beam 1410 frombeam conditioning optics 1408 may be transmitted to a variable opticaldivider (e.g., including a beam splitter such as 216 shown in FIG. 2A),indicated generally as 1412, of subsystem 1400 wherein conditioned beam1410 is divided into a data beam, indicated generally as 1414, and areference beam, indicated generally as 1416. Data beam 1414 may betransmitted to and received by fiber coupling optics, indicatedgenerally as 1418, and focused through a shutter 1420 onto the input endof a fiber optic connector, indicated generally as 1422. Similarly,reference beam 1416 may be transmitted to and by received by fibercoupling optics, indicated generally as 1424, and focused through anoptional shutter 1428 onto the input end of a fiber optic connector,indicated generally as 1430. Data beam 1414 may then be transmittedthrough a fiber optic cable, indicated generally as 1434, which isterminated at one end by fiber optic connector 1422 and at the other endto a fiber optic connecter, indicated generally as 1438, whichrepresents the end of subsystem 1400. Similarly, reference beam 1416 maybe transmitted through a fiber optic cable, indicated generally as 1442,which is terminated at one end to fiber optic connector 1430 and at theother end to a fiber optic connecter, indicated generally as 1446.Cables 1434 and/or 1442 may each also be divided in two with matingconnectors between each pair of cables.

In an alternative embodiment, a single set of fiber coupling optics1418, shutter 1420, fiber optic cable 1434 and collimation optics 1350may be used in optical path 1410 between the beam conditioning optics1408 and the variable optical divider 1412 to achieve the same orsimilar results. In this alternative embodiment, a second set of cable1442, shutter 1428, fiber coupling optics 1424, and collimation optics1452 would be omitted. Shutters 1420 and/or 1428 may also be positionedbefore respective fiber coupling optics 1418 and/or 1424, beforevariable optical divider 1412, etc.

As further shown in FIG. 19, data beam 1414 may be transmitted fromcable 1434 via connector 1438 to collimation optics, indicated generallyas 1450, while reference beam 1418 may be transmitted from cable 1442via connector 1446 to collimation optics, indicated generally as 1452.Collimation optics 1450 and 1452 represent the end of subsystem 1400.The collimated data and reference beams 1454 and 1456 are thentransmitted from respective collimation optics 1450 and 1452 torespective data beam path optics, indicated generally as 1460, andreference beam path optics, indicated generally as 1466. The resultingdata beam 1470 and reference beam 1474 may then be transmitted so as tointerfere and thus form holograms which are recorded by holographicmedium 106.

All documents, patents, journal articles and other materials cited inthe present application are hereby incorporated by reference.

Although the present invention has been fully described in conjunctionwith several embodiments thereof with reference to the accompanyingdrawings, it is to be understood that various changes and modificationsmay be apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

1. A unitized subassembly comprising: a spatial light modulator; adetector array; a generally cube-shaped beam splitter having a firstface adjacent to and opposing the spatial light modulator, a second faceperpendicular to the first face, and third and fourth opposite faceswhich are perpendicular to the second face; and means for associatingthe spatial light modulator and the detector array to the beam splitterso that the spatial light modulator and the detector array are opticallyand mechanically aligned with respect to each other, wherein theassociating means comprises a detector array frame for mounting thedetector array adjacent to and opposing the second face; and a pluralityof mounting blocks, wherein each mounting block is attached to one ofthe third and fourth faces.
 2. The subassembly of claim 1, wherein thedetector array comprises a camera.
 3. The subassembly of claim 2,wherein the beam splitter comprises a polarizing beam splitter.
 4. Thesubassembly of claim 1, wherein the detector array comprises a camera.5. The subassembly of claim 4, wherein the beam splitter comprises apolarizing beam splitter.
 6. The subassembly of claim 1, whereinassociating means comprises a bonding agent comprising microspheres tojoin the spatial light modulator to the beam splitter.
 7. Thesubassembly of claim 1, wherein the associating means comprises acircuit board for powering or activating the spatial light modulator. 8.A holographic data storage system which comprises the subassembly ofclaim
 1. 9. A subassembly comprising: a spatial light modulator; adetector array; a beam splitter having a first face and a second faceperpendicular to the first face; a spatial light modulator mountingstructure for housing the spatial light modulator; and a detector arrayframe; wherein the spatial light modulator mounting structure associatesthe beam splitter with the spatial light modulator so that the firstface is adjacent to and opposing the spatial light modulator; whereinthe detector array frame associates the beam splitter with the detectorarray so that the detector array is adjacent to and opposing the secondface, and to thereby form a unitized subassembly with the spatial lightmodulator mounting structure; wherein the unitized subassembly formedoptically and mechanically aligns the spatial light modulator and thedetector array with respect to each other; wherein the beam splitter isgenerally cube-shaped and has third and fourth opposite faces which areperpendicular to the second face, the subassembly further comprising aplurality of mounting blocks, each mounting block being attached to oneof the third and fourth faces.
 10. The subassembly of claim 9, whereinthe detector array comprises a camera.
 11. The subassembly of claim 9,wherein the beam splitter comprises a polarizing beam splitter.
 12. Thesubassembly of claim 9, wherein the spatial light modulator mountingstructure comprises a circuit board for powering or activating thespatial light modulator.
 13. A holographic data storage system whichcomprises the subassembly of claim
 9. 14. The subassembly of claim 9,wherein the mounting structure comprises a circuit board for powering oractivating the spatial light modulator.
 15. The subassembly of claim 9,wherein the detector array comprises a camera.
 16. The subassembly ofclaim 15, wherein the beam splitter comprises a polarizing beamsplitter.